[llvm-dev] Question about : lprofValueProfNodes

[Vedant Kumar via llvm-dev]

> On Dec 19, 2017, at 5:16 PM, Moshtaghi, Alireza <Alireza.Moshtaghi at netapp.com> wrote:
> 
> Thank you
> So it does not seem to be relevant for what I’m trying to do.
> I’m doing something unconventional.
> The objective is to implement PGO and code coverage on a system that does not exit and does not have any file io, or any of  stdc libraries that libclang-profile is using. (more like a kernel)
> So what I’m trying to do is instead of calling __llvm_profile_write_file () from the application, read the sections __llvm_prf_data, __llvm_prf_names, __llvm_prf_cnts and __llvm_prf_vnds after the critical tasks are done and transfer them to outside of the system.

You may already have worked this out, but for completeness I'll mention that this is typically done by:
1. Allocating a buffer big enough to contain all the data (see __llvm_profile_get_size_for_buffer_internal),
2. Filling out the buffer (see __llvm_profile_write_buffer_internal), and
3. Transferring the buffer to some host machine.


> Then dump these sections in a char * array in a c file and attribute them to be in the associated section.

I'm not sure I follow -- why is a C file needed?

Once you have the buffer from step 3 on the host machine, you've got a well-formed raw profile. You should be able to fwrite() it to e.g 'default.profraw' and test that it's a valid profile with llvm-profdata.


>  Then compile that file with –u__llvm_profile_runtime to create an executable that calls __llvm_profile_write_file to dump my profraw data.
>  
> Sofar, I’m able to do the mechanics but seems like something is going wrong because because fprofile-instr-use does not find profile data for the source files.

What error are you getting exactly? Have you confirmed that your profile is valid?

vedant

>  
> Any suggestion?
>  
> Thanks
> A
>  
> From: <vsk at apple.com> on behalf of Vedant Kumar <vsk at apple.com>
> Date: Tuesday, December 19, 2017 at 11:32 AM
> To: "Moshtaghi, Alireza" <Alireza.Moshtaghi at netapp.com>
> Cc: llvm-dev <llvm-dev at lists.llvm.org>, Xinliang David Li <davidxl at google.com>
> Subject: Re: [llvm-dev] Question about : lprofValueProfNodes
>  
> Hi,
>  
>  
> On Dec 19, 2017, at 10:26 AM, Moshtaghi, Alireza via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>  
> Hi
> This array is defined in compiler-rt: InstrProfilingValue.c but I can’t find where it is used?
>  
> It's used in allocateOneNode(). Incrementing the current vnode pointer gives a fresh node (possibly backed by the shared pool).
>  
> 
> 
> And the comment on it does not say much about why we need it either.
>  
> It's used to avoid calling malloc(), which David (CC'd) found to be a performance improvement.
>  
> best,
> vedant
> 
> 
> Can someone explain why we need this and where it is used?
>  
> /* A shared static pool in addition to the vnodes statically
> * allocated by the compiler.  */
> COMPILER_RT_VISIBILITY ValueProfNode
>     lprofValueProfNodes[INSTR_PROF_VNODE_POOL_SIZE] COMPILER_RT_SECTION(COMPILER_RT_SEG INSTR_PROF_VNODES_SECT_NAME_STR);
>  
> Thanks
> A
>  
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> 

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